The long term objective of this research is to better understand how the composition and material properties of an extracellular matrix are influenced by the mechanical forces to which that matrix is subjected. Experiments are proposed to investigate the structure, metabolism and role of proteoglycan constituents in the extracellular matrix of a normal dense regular connective tissue, tendon. The transformation of a region of bovine deep flexor tendon into a compression-resistant structure containing increased amounts of large proteoglycans and a fibrocartilaginous organization will be documented biochemically and histologically, from the time that a newborn animal begins to walk until the region is fully developed. In addition, pieces of bovine flexor tendon will be maintained in culture and subjected to controlled long term cyclic compressive loading utilizing a device designed and built by the investigators. The ability of compressive loading to maintain synthesis of large, cartilage-like proteoglycans in tissue from regions of tendon that make such molecules in vivo will be tested, using biochemical and immunological assays. In addition, the response to compressive loading of tendon that does not normally show this fibrocartilaginous differentiation, and of young tissue that has not yet shown this differentiation, will be assessed. The hypothesis to be tested is that fibroblasts of tendon respond to compressive forces by synthesizing specific molecular components, such as large proteoglycans, that are responsible for the appropriate material properties of that matrix. The regulation of synthesis of both large and small proteoglycans by application of tensile forces to tendon in culture will also be investigated. Molecular probes developed to recognize the core protein of large proteoglycans in cartilage will be assessed for hybridization to mRNA in cells from the tensional and compressional regions of tendon The relative expression of this specific RNA will be determined in each tendon region. In addition, the appearance and localization of cells responsible for producing the cartilage- like molecules under compressive loading will be determined, using in situ hybridization techniques. Restoration of function to a damaged tendon is a difficult medical problem. Investigation of the capability of young and adult tendon fibroblasts to modulate proteoglycan synthesis and thus alter the functional properties of their extracellular matrix is relevant both to basic cell and tissue biology as well as to development of more rational surgical procedures and therapies.